Coating materials to protect and improve the surface qualities of articles such as plastic or metallic (i.e., solid) fasteners and other assembly articles have become widespread and widely accepted, particularly in the automotive industry where stringent requirements generally exist. Among the requirements are a surface that exhibits a low but consistent coefficient of friction, that includes a coating which strongly adheres to the substrate and that includes additives for purposes such as corrosion protection and coloration. Numerous coatings and treatments have been developed and used for such purposes, and many of these remain important. However, especially in the automotive industry, performance requirements for such articles have become increasingly stringent. Some materials, such as hexavalent chromium, are no longer acceptable or are being phased out. As the performance, environmental, health, safety and disposal (end of life) requirements become more stringent, the point has come where existing compositions are unable to meet all of the requirements. The prevention of corrosion has also become a major concern, particularly in the automotive industry. At the same time as requirements such as these are becoming more stringent, the articles are still required to meet the same usefulness standards as have historically been required such as, for example, providing a consistent coefficient of friction so that torque requirements can be consistently measured and met, thus allowing a precise determination of the actual lubricant content.
Coating articles such as a bolt with a corrosion resistant coating has obvious importance. Steel bolts are common but are not effective if severely corroded. Coating methods include phosphatizing, phosphatizing followed by painting or oil immersion, plating (e.g. electroplating, electroless plating, mechanical plating, or galvanizing) and plating followed by painting. For fasteners with tight dimensional tolerances electroplating or electroless plating is often a preferred method of preventing corrosion. In particular, zinc or zinc alloy electroplating, in which the zinc corrodes preferentially and is sacrificial to steel corrosion, is a cost effective corrosion protection method. However, zinc plating has generally been accompanied by formation of a conversion coating by, e.g., chromating (forming a film from hexavalent chromium), or passivation (forming a film from trivalent or non-chromium solutions). Recently, it has become common practice to treat conversion coated, electroplated zinc fasteners with a seal to extend corrosion protection. Seals are generally based upon siccative and/or curable solutions that include inorganic films such as silicates, or mixtures of silicates with silica, or organic films, such as acrylates, urethanes, or mixtures of acrylates and urethanes; or mixtures of inorganic and organic materials.
The tightening of a bolt or a nut to produce a clamping load, or the insertion of a rivet into a hole is an important component of assembly of a multitude of mechanical assemblies, such as automobiles. The reliability, safety and quality of bolted assemblies are affected by the level and stability of fastener tension. Turning the head of a threaded fastener, or turning the nut onto a threaded fastener achieves fastener tension. It is commonly accepted that overall tightening torque is a combination of 1) the friction of the threads; 2) the tightening force imparted to the bolt; and 3) the friction between the contact surface of the assembly, the bearing surface, and the underhead of a bolt and/or between the bearing surface of the assembly and the underhead of the nut.
Today many assembly steps are carried out by robots. The robots may be programmed to determine when a part, a bolt for example, is sufficiently tightened, based primarily on the torque measured by sensors. If lubrication applied to a bolt is too little (or if the coefficient of friction is too great), then the robot may not fully tighten the bolt, because the sensed torque will meet the pre-programmed torque limit as a result of the excess friction, not because the bolt is actually fully tightened. Conversely, if lubrication applied to a bolt is excessive (and the coefficient of friction is too low), then the robot may over-tighten the bolt and damage the assembly, for example, by snapping off the head of the bolt or stripping out the threads, because the sensed torque will not meet the pre-programmed torque limit as a result of the too-low friction. Thus, attaining adequate but not excessive lubrication is a problem constantly facing those involved in assembly operations. Because the robotic systems include sensors to determine torque, these sensors and the electronic controls are programmed to use various equations known in the art to correlate torque with friction and thereby with tightening determinations. These systems must necessarily make assumptions about the flatness (or smoothness) of surfaces, about the coefficient of friction, about the clamping load applied to a bolt or other part, about wear rates for lubricants, conversion coatings and substrates, and about other variables. All of these assumptions are used to address the problem of determining when a fastener has been sufficiently tightened, for example, and the assumptions rely on consistent, relatively uniform parts having consistent characteristics.
Because friction is an important part of the process of tightening a fastener, e.g., a threaded bolt and nut combination, the control of friction by lubrication of the contacting surfaces is important. Lubrication sufficiency may be determined by measurement of the friction, or coefficient of friction, resulting from use of the lubricant applied between surfaces moving with respect to each other. Lubrication regimes may be defined by a Stribeck curve where coefficient of friction is plotted as a function of sliding speed for a combination of two surfaces, referred to as a tribological pair, and an intervening lubricant. Boundary lubrication is the situation if the sliding speed is low and the loading force is totally carried by asperities in the contact area, protected by adsorbed molecules of the lubricant and/or a thin lubrication and/or oxide layer. The interactions of boundary lubricated surfaces and the relationship of shear strengths of lubricants and surface asperities to overall lubricity are known to relate to wear of the lubricant and to play a role in boundary lubrication.
Boundary lubrication situations exist with machine bearings when they are being started or stopped and the velocity is not high enough to create a hydrodynamic regime. Other examples include contact between gear teeth, turbine wicket gates and other slow moving equipment such as hydraulic reciprocation in lifting operations or with shock absorbers or with struts used, for example, to control opening and closing of automobile hood or trunk lids. In boundary lubrication, atomically flat surfaces are separated by a few molecular layers of lubricant, and the behavior of the interface becomes qualitatively different from the more familiar case of bulk viscosity which is traditionally associated with lubricants. Boundary lubrication may be important where surfaces are not atomically flat, particularly at the sites of any asperity or where slightly differing geometries result in the closer approach of relatively moving parts in certain areas or regions.
Stick-slip lubrication situations exist with parts moving with respect to each other when separated by a lubricant in which the parts move in a “stop-start” cycle. Lubricant additives may be included to avoid or reduce this phenomenon.
To help control the torque during tightening, the fasteners that have been prepared by, e.g., zinc plating, conversion coating, and possibly sealing may be treated with a lubricating solution such as diluted FUCHS LUBRITECH® Gleitmo 605 or diluted Valsoft PE-45 from Spartan Adhesives and Coatings Co. Such lubrication coatings are often called dry film lubricants. The lubricant may also be combined with a seal layer, in which the resulting coating may be referred to as an integral lubricant seal. In these lubrication processes, the lubricant is uniformly distributed in the single lubricious coating.
Some typical prior art treated metal parts are schematically illustrated in FIGS. 1-4. FIG. 1 schematically depicts a prior art metal article 100 including a metallic substrate 10, a zinc or zinc alloy layer 12, an anticorrosion layer 14 and a lubricant coating 16. The substrate 10 may be any suitable metal article, such as a fastener, e.g., a bolt, nut, screw, etc., which may be made of steel, for example. In such prior art embodiments, the zinc or zinc alloy (e.g., zinc-iron, zinc-cobalt, zinc-nickel) layer 12 may be, for example, a hot-dip galvanized or electroplated layer applied to the substrate 10. In some prior art embodiments, the anticorrosion layer 14 would be a hexavalent chromium conversion coating, applied directly to the zinc or zinc alloy layer 12. The lubricant layer 16 may contain any known lubricant for such uses. A seal layer (not shown) may be formed over the lubricant, or the lubricant may be an integral part of the seal layer, in which case the lubricant layer 16 would be a seal/lubricant layer. In this example, the lubricant is uniformly distributed in the single lubricious coating.
FIG. 2 schematically depicts a prior art metal article 200 including a metallic substrate 10, a zinc or zinc alloy layer 12, an anticorrosion layer 14, a seal layer 18 and a lubricant coating 16. The substrate 10 may be any suitable metal article, such as a fastener, e.g., a bolt, nut, screw, etc., which may be made of steel, for example. In such prior art embodiments, the zinc or zinc alloy (e.g., zinc-iron, zinc-cobalt, zinc-nickel) layer 12 may be, for example, a hot-dip galvanized or electroplated layer applied to the substrate 10. The anticorrosion layer 14 again may be a hexavalent chromium conversion coating, applied to the zinc layer 12. A seal layer 18 is applied over or onto the anticorrosion layer 14. The lubricant coating 16 may contain any known lubricant for such uses. In this example, the lubricant is uniformly distributed in the single lubricious coating.
FIG. 3 schematically depicts a prior art metal article 300 including a metallic substrate 10, a zinc or zinc alloy layer 12, an anticorrosion layer 20, a seal layer 18 and a lubricant coating 16. The substrate 10 may be any suitable metal article, such as a fastener, e.g., a bolt, nut, screw, etc., which may be made of steel, for example. In such prior art embodiments, the zinc or zinc alloy layer 12 may be, for example, a hot-dip galvanized or electroplated layer applied to the substrate 10. The anticorrosion layer 20 in this prior art example may be a passivation material free of hexavalent chromium, applied to the zinc layer 16. The passivation material free of hexavalent chromium may include, for example, trivalent chromium, an electroless nickel, or an electroplated nickel-phosphorus (NiP) alloy, deposited on the zinc layer 12. The seal layer 18 is applied over or onto the anticorrosion layer 20. The lubricant layer 16 may contain any known lubricant for such uses. As with the examples of FIGS. 1 and 2, the seal layer 18 may be applied over the lubricant layer 16. In this example, the lubricant is uniformly distributed in the single lubricious coating.
FIG. 4 schematically depicts a prior art metal article 400 including a metallic substrate 10, a zinc or zinc alloy layer 12, an anticorrosion layer 14 and an integral seal layer 22. The substrate 10 may be any suitable metal article, such as a fastener, e.g., a bolt, nut, screw, etc., which may be made of steel, for example. In such prior art embodiments, the zinc or zinc alloy layer 12 may be, for example, a hot-dip galvanized or electroplated layer applied to the substrate 10. The anticorrosion layer 14 applied to the zinc layer 16 in this prior art example may be the hexavalent chromium layer 14 shown, or it may be a passivation material free of hexavalent chromium, such as the anticorrosion layer 20 shown in FIG. 3, or any other suitable passivation or anticorrosion material. The passivation material free of hexavalent chromium may include, for example, trivalent chromium, an electroless nickel, or an electroplated nickel-phosphorus (NiP) alloy, deposited on the zinc layer 12. In this alternative, there is an integral seal layer 20 in which the lubricant is an integral part of a seal layer. In this example, the lubricant is uniformly distributed in the single lubricious coating.
In all of the foregoing prior art examples (e.g., as described with respect to FIGS. 1-4), and in the prior art generally, the lubricant layer or integral seal layer is a uniform lubricious layer in which the lubricant is evenly and uniformly distributed. Such lubricants are generally dry to the touch antifriction coatings, and these include and are sometimes referred to as dry film lubricant, organic lacquer lubricant or an integral lubricant. The dry film lubricant is dry after application and heating and the resulting lubrication layer is essentially only lubricant, so by definition is uniformly distributed. The organic lacquer lubricant may contain an organic binder and a solid lubricant. The organic binder may be, e.g., wax with resins, and the solid lubricant. The integral lubricant coating is a mixture of lubricant with other materials, such as aesthetic (e.g., color) or corrosion resistant or combinations, in which particles of the lubricant are uniformly distributed. In both the organic lacquer lubricant and the integral lubricant, the lubricant material may be, e.g., particles of a polymer such as polyethylene, PTFE, graphite or another material such as molybdenum sulfide (MoS2), for example. When any additional lubricant is used, it is generally the most lubricious and it is applied to the outermost surface of the part.
In all of the foregoing examples, and in the prior art generally, the problem of providing adequate but not excessive lubrication for a wide variety of parts having various geometries and surface qualities and features, has persisted. Although many lubricants are known and widely used, a need remains for improved lubricants, particularly for parts such as fasteners that are subjected to large pressure, torque and frictional forces and where such forces are frequently applied by robots.
While applying a lubricant to the exterior of an object produces the advantage of reducing friction it is very difficult to achieve uniformity of friction with the single uniform layer of lubricant unless the coating containing the lubricant is thick enough to not be worn through during the period when lubrication is necessary. However, thick coatings are not useful for threaded fasteners, as they will affect the dimensions of the male and female threads. As with other objects, thick coatings may also deleteriously affect dimensional tolerances. With thin lubricious coatings containing uniform lubricant, the surfaces of the sliding objects are very close and the lubricating media is very thin, and this combination means that small imperfections in the surface, asperities, and/or uneven distribution of pressure over the surface due to geometric differences, will produce variations in wear such that some regions will have sufficient lubricant, while in other regions the lubricant layer will be worn through or broached. When such wear or broach occurs, the apparent overall coefficient of friction changes for the sliding surfaces, resulting in problems associated with the change in friction, and where the lubricant layer has been completely worn away, the underlying layers may also be damaged, resulting in possibly deleterious effects, for example, an increased possibility of corrosion, during the lifetime of the substrate.
For all these reasons, problems remain and improvements in lubricious coatings, particularly for mass produced but critically important parts such as fasteners, are very much needed.